Symptoms are similar to classical disease but current vaccine is no protection

by Michael Smith Michael Smith North American Correspondent, MedPage Today
September 21, 2015

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Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

Bordetella parapertussis, a relatively rare and antigenically distinct cousin of the pathogen B. pertussis, can present a clinical picture very similar to classical whooping cough.

Vaccination against B. pertussis does not protect against infection caused by B. parapertussis.

SAN DIEGO -- When is whooping cough not whooping cough?

When it's caused by Bordetella parapertussis, a relatively rare cousin of the pathogen B. pertussis that is the usual cause of the disease, Minnesota researchers said here.

But although the causes are different, the clinical picture of the disease seen in a late-2014 outbreak in the state was very similar to classical whooping cough, according to Vytas Karalius, MPH, a medical student at the Mayo Medical School in Rochester, Minn.

While the two pathogens are related, they are "antigenically very different," he said, so the acellular vaccine would not be expected to be protective. And, in fact, the patients in the Minnesota outbreak were all vaccinated against B. pertussis but still became ill.

The outbreak was discovered, in a sense, by accident, according to the study's senior author Robin Patel, MD, director of the Mayo Clinic's Infectious Diseases Research Laboratory.

Her lab has a polymerase chain reaction (PCR) test that picks up both pathogens and can distinguish between them. The lab is a reference facility for the U.S. and usually sees a steady but low incidence of B. parapertussis.

In the fall of 2014, they saw a spike in parapertussis in southeastern Minnesota and a similar increase nationwide. To try to figure out what was going on, they delved into medical records of the Minnesota patients.

But it seems likely that the local outbreak -- studied because they had access to records -- was just part of something larger, Patel said. "We're still digging into some of that data," she said.

From August onward in Minnesota, Karalius reported, the researchers were able to identify 31 cases, with 25 of them in the last 3 months of the year. The patients in that last group had an average age of 5.9 years, with two younger than a year and one as old as 11 but no neonates.

There were no obvious epidemiological links, he told MedPage Today, although there were two pairs of siblings as well as five patients whose parents reported they had been exposed to pertussis.

The clinical picture was "almost indistinguishable" from classical whooping cough, he said:

40% of the patients had post-tussive vomiting

40% had coryza

32% had apnea or sleep disturbances

12% reported a sore throat

And none had fever

The duration of cough before visiting a doctor ranged from a few days to 8 weeks, he said, and the records showed that several of the children had the classic "whoop" after a coughing bout, he said.

In a pertussis outbreak in Minnesota in 2012, she said, most cases involved B. pertussis but her lab was able to establish that some patients in fact were affected by B. parapertussis.

So it's important when studying whooping cough, she said, to know how an outbreak is diagnosed -- if it's done on the basis of clinical signs and symptoms, the cause of some cases might not be B. pertussis at all. And that in turn could undermine theory about the vaccine.

On the other hand, it's probably premature to start ringing the alarm bells over B. parapertussis, commented Keith Klugman, MD, PhD, of the Bill & Melinda Gates Foundation in Seattle, who was not involved in the study but who moderated a session at which it was presented.

"It's interesting, but it would be early days to have a specific parapertussis concern," Klugman told MedPage Today.

"Clearly there is concern around pertussis but we need more time to evaluate parapertussis as a significant threat," he added. "Pertussis overwhelms parapertussis as a disease."

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